Since my bullish recommendation a week ago, Groupon has lost almost 11% of its value. And given the company's struggles with rising costs and competition for advertising dollars, things won't rebound anytime soon. At the time, I said:

From my vantage point, there is at least 40% upside in shares of Groupon in the next 6 to 12 months. During that span the stock should reach $9 on the basis of margin expansion, cost-reduction and free cash flow growth.

Clearly, I was wrong. The most important lesson learned was this: Though the market may be inefficient, it's not always wrong. There's a difference.

Matching Groupon's growth forecast with that of other Internet/retail companies such as Amazon (AMZN) (price-to-earning ratio of 157) and Facebook (FB) (P/E of 35) was not enough. My mistake was looking at Groupon's forward P/E of 25 and thinking that it implies value.

Instead, I should have looked at Groupon's underlying operation and the company's prospects to make money. As it stands, management's plan to transform the company is falling short of expectations, despite what its recent 26% revenue growth rate might suggest.

The other thing is, while Groupon's gross billings have improved, that alone does not signal the level of strength that this metric would ordinarily indicate. Gross billing is the metric that reflects the total dollar value of customer purchases of goods and services, excluding applicable taxes and net of estimated refunds.